European application EP13198913.9, unpublished at the priority date of the present invention, discloses light-sensitive photopolymers which comprise polyurethane matrix polymers, an acrylate-based writing monomer and also photoinitiators comprising two coinitiators and a dye. The uses of photopolymers are decisively determined by the refractive index modulation Δn produced by holographic exposure. In holographic exposure, the interference field of signal light beam and reference light beam (that of two plane waves in the simplest case) is mapped into a refractive index grating by the local photopolymerization of writing monomers such as, for example, high-refractive acrylates at loci of high intensity in the interference field. The refractive index grating in the photopolymer (the hologram) contains all the information of the signal light beam. Illuminating the hologram with only the reference light beam will then reconstruct the signal. The strength of the signal thus reconstructed relative to the strength of the incident reference light is called the diffraction efficiency, DE in what follows.
In the simplest case of a hologram resulting from the superposition of two plane waves, the DE is the ratio of the intensity of the light diffracted on reconstruction to the sum total of the intensities of diffracted light and nondiffracted light. The higher the DE, the greater the efficiency of a hologram with regard to the amount of reference light needed to visualize the signal with a fixed brightness.
In order that a very high Δn and DE may be realized for holograms, the matrix polymers and the writing monomers of a photopolymer formulation should in principle be chosen such that there is a very large difference in their refractive indices. One possible way to realize this is to use matrix polymers having a very low refractive index and writing monomers having a very high refractive index. Suitable matrix polymers of low refractive index are, for example, polyurethanes obtainable by reaction of a polyol component with a polyisocyanate component.
In addition to high DE and Δn values, however, another important requirement for holographic media from photopolymer formulations is that the matrix polymers be highly crosslinked in the final medium. When the degree of crosslinking is too low, the medium will lack adequate stability. One consequence of this is to appreciably reduce the quality of holograms inscribed in the media. In the worst case, the holograms may even be subsequently destroyed.
It is further very important, in particular for the large scale industrial production of holographic media from photopolymer formulations, that the photosensitivity be sufficient to achieve large-area exposure with any given source of laser light without loss of index modulation. Particularly the choice of a suitable photoinitiator here is of decisive importance for the properties of the photopolymer.
However, holographic exposure using a continuous source of laser light comes up against technical limits in the case of large-area exposure, since efficient formation of the hologram will always require a certain minimum of light per unit area and the technically available laser power is limited. Large-area exposures at a comparatively low dose of radiation additionally require long exposure times which in turn impose very high requirements on the mechanical damping of the exposure set-up to eliminate vibration.
A further possible way to achieve large-area exposure of holograms consists in using very short pulses of light, for example from pulsed lasers or continuous wave lasers in conjunction with very fast shutters. Pulse durations with pulsed lasers are typically 500 ns or less. Pulse durations with continuous wave lasers and very fast shutters are typically 100 μs or less. In effect, the same amount of energy can be introduced here as with continuous lasers in seconds. Holograms can be written in this way dot by dot. Since pulsed lasers or fast optical shutters are technically available and an exposure set-up of this type has very low requirements with regard to mechanical damping to eliminate vibration, this amounts to a good technical alternative to the above-described set-ups involving continuous lasers for large-area exposure of holograms.
The photopolymers known from European application EP13198913.9 are generally usable for writing holograms with pulsed lasers but a further increase in sensitivity of the photopolymer is desirable for broad industrial utility.